At present, circuit-switched (CS) networks, such as global system for mobile communication (GSM) networks and code division multiple access (CDMA) networks, dominate the mobile communication network; and operators have established complete and abundant service platforms based on the CS networks. An IP multimedia subsystem (IMS) that supports various services like audio, video, and data, is regarded as a core technique of a next generation network that integrates mobile and fixed communication networks. The IMS network structure is too complex to complete all-round deployment within a short period of time; therefore, the CS network will coexist with the IMS network for a certain period. However, operation costs will definitely increase for operating a CS service platform and an IMS service platform at the same time. To reduce the operation costs, it is hoped that functions of the CS network service platform can be transferred to the IMS network to integrate the service platforms. An IMS centralized service (ICS) project is proposed to meet the requirement of the service platform integration; and the main task is to realize that user equipment (UE) establishes an IMS call by bearing audio media through a CS network, where the CS bearer is realized by establishing a CS call between the UE and a service centralization & continuity application server (SCC AS). The SCC AS completes an adaptation from CS domain signaling to IMS domain session initiation protocol (SIP) signaling Messages between the UE and the SCC AS can be transmitted through a packet switched service control signaling channel or a circuit switched service control signaling channel. The packet switched service control signaling channel is also known as a Gm interface, and the circuit switched service control signaling channel is also known as an Il interface. Comparing with the Il interface, the Gm interface has advantages of transferring more information and abundant contents. It is suggested in a standard that when both a Gm and an Il interface are available, the Gm interface is preferred as a channel for session control message transmission. And the Il interface is selected as a channel for session control message transmission when there is only a CS domain connection.
Explicit call transfer (ECT), as a supplementary service of mobile communication, provides a method for transferring a communication party during an existing call for users. The ECT may be either a blind transfer or a consult transfer. When a user A is communicating with a user B and a user C at the same time, and the user B and the user C are in a state of call holding, the user A wants the user B to communicate with the user C, so the UE-A initiates a consult ECT process, details of which are shown as FIG. 1. After a step 21 is executed, the UE-B starts session connection with the UE-C, the connection between the UE-A and the UE-B, and the connection between the UE-A and the UE-C are broken.
In the above process, the UE-A is known as a transferor, which initiates an ECT request; the UE-B is known as a transferee, which is configured to receive the ECT request and execute an ECT service; and the UE-C is known as a transfer target, which is a calling target of the transferee. In order to achieve the integration of the ECT service between a CS network and an IMS network, in the ICS project, methods for a consult ECT using an IMS centralized service user equipment (ICS UE) as a transferor or as a transferee have already been provided. However, currently no method has been provided for using an ICS UE as a transfer target to realize a consult ECT. That is, the standard for the consult ECT service in the ICS project is not complete, manufacturers cannot produce relevant equipment according to the standard, and thus the ICS consult ECT service cannot be put into practical use.